Conventionally, in a vector control of a multiphase rotating electric machine (e.g., electric motor), an optimal dq-axis current instruction value is calculated based on a preset or desired condition. For example, the controller disclosed in Japanese Patent No. 4462207 changes a d-axis current instruction value and a q-axis current instruction value based on a target torque of the electric machine, to limit/prevent a current ripple at a field-weakening control time. That is, the dq-axis current amplitude and current phase are changed in such manner.
In general, when a d-axis inductance and a q-axis inductance are different from each other in a magnet-embedded-type rotating electric machine, a reluctance torque is generated based on the difference of the two inductances. Even in a magnet-exposed-type rotating electric machine, a nominal amount of inductance torque is generated in some cases. Therefore, the total torque output from the rotating electric machine is a combination of the magnet torque and the reluctance torque.
Based on the above, when driving a multiphase rotating electric machine that has two sets of windings with two power converters supplying power to the respective driving systems/windings, a two-system driving mode may be switched to a one-system driving mode when one of the systems fails. In the two-system driving mode, a reluctance torque is generated based on mutual inductance between the two sets of windings, while in the one-system driving mode, no reluctance torque is generated based on mutual inductance. Based on an assumption that the two systems have the same current amplitude, the magnet torque in the one-system driving mode has half the size of the magnet torque in the two-system driving mode, and the reluctance torque in the one-system driving mode is smaller than the reluctance torque of the two-system driving mode. As a result, the total torque of the one-system driving mode is smaller than half the total torque of the two-system driving mode. In other words, the total torque of the one-system driving mode is smaller than the torque of one of the systems in the two-system driving mode.
The current phase that maximizes the total torque is different in the two-system driving mode and in the one-system driving mode, even with the same current amplitude. In other words, a ratio between the d-axis current and the q-axis current is different in the two-system driving mode and in the one-system driving mode. As such, the optimal current phase in the two-system driving mode may not be usable for maximizing the total torque in the one-system driving mode.